In wireless communication networks, users may move about geographically freely, at least within the coverage area of the wireless communication network. There are different types of so-called generations of communication networks, e.g. 2G, 3G and 4G. These different generations employ different techniques, e.g. 2G wireless communication networks are also called Global System for Mobile Communications, GSM, is based on Time Division Multiple Access, TDMA; 3G wireless communication networks are also called Universal Mobile Telecommunications System, UMTS, is based on Wideband Code Division Multiple Access, WCDMA; and 4g wireless communication networks are also called Long Term Evolution, LTE, is based on Orthogonal Frequency Division Multiplex, OFDM. These different wireless communication systems have both similarities and differences.
In 3G or WCDMA wireless communication systems, hereinafter referred to as WCDMA systems or WCDMA networks, mobile stations or User Equipments, UEs, may be in different operation modes depending on whether the UE is camping on a cell or is actively engaged in a service offered by the CDMA network. A cell is a coverage area of a Radio Base Station, RBS, and by camping on a cell means that the UE is connected to the RBS having the cell as its coverage area and wherein the UE is idle, i.e. not engaged in a service.
A UE may enter a cell and thus becoming connected to the RBS either currently being active or engaged in a service, e.g. a phone call, or as being switched on but not in active use by the user of the UE. When in the cell, or when connected to the RBS, the user may determine to start using a service, e.g. making a phone call or looking at an Internet site. Depending on which “stage of activity” the UE is in, the UE is in different so-called Radio Resource Control, RCC, states. In WCDMA there are several different states, e.g. IDLE, URA_PCH, CELL_PCH and CELL_FACH. URA is short for Utran Registration Area and PCH is short for Paging Channel, wherein Utran is short for UMTS Terrestrial Radio Access Network. FACH is short for Forward Access Channel.
Depending on which RRC state the UE is in, the UE may use different uplink and downlink channels. In IDLE, URA_PCH, CELL_PCH, and CELL_FACH state, the channel used in the uplink is the Random Access CHannel, RACH, however, E-DCH possible from 3rd Partnership Project, 3GPP, Release 8. The RACH channel is used for signalling in all four RRC states mentioned above and also for transmitting data in uplink in CELL_FACH state.
As with all channels, the capacity of RACH is not infinite. The RACH channel has several limitations. There is a limited number of access slots available for preambles. In addition there is a fixed number of preamble signatures. If more than one UE (randomly) chooses a specific preamble signature for transmission in a particular slot there will be a collision. RACH decoding possibilities in the RBS (decoding of both the preamble itself and of the RACH message that follows the preamble) might be a further limitation. When any one of these or other possible limitations is reached, the quality of the RACH channel deteriorates leading to higher latency and higher Block Error Rate, BLER.